1. Field of the Invention
The present invention relates generally to optical recording and/or reproducing apparatus, and more particularly, is directed to improvements in an apparatus operative to cause a light beam or light beams to impinge upon a disc-shaped record medium for recording information thereon or reproducing information therefrom.
2. Description of the Prior Art
In an optical disc player for optically recording an information signal on or optically reproducing an information signal from an optical disc-shaped record medium having a center hole and a recording area on which a spiral record track is composed to surround the center hole, an optical head device is provided for constituting an optical arrangement for causing a light beam to impinge upon the recording area of the optical disc-shaped record medium rotating with a rotation center disposed at the center hole so as to form the spiral record track corresponding to a recording information signal on the recording area or to read an information signal recorded in the spiral record track on the recording area.
In the optical head device, for example, a laser light beam produced by a light beam generating portion is collimated by a collimator lens and enters into an objective lens to be focused thereby to impinge upon the optical disc-shaped record medium, and then a reflected light beam emanating from the optical disc-shaped record medium is directed through the objective lens to a beam splitter to be changed in a direction of its optical axis thereby so as to enter into a photodetecting portion. A reproduced information signal, a focus error signal and a tracking error signal are produced based on a detection output of the reflected light beam from the photodetecting portion, and a focus servocontrol operation for maintain correct focus of the light beam projected onto the optical disc-shaped record medium and a tracking servocontrol operation for maintain the light beam in correct tracking relation to each turn of the spiral record track on the optical disc-shaped record medium are performed in accordance with the focus error signal and the tracking error signal, respectively.
As for the production of the tracking error signal, various signal producing systems including the so-called "three-beam system" have been known. In the so-called "three-beam system", a light beam produced by a light beam generating portion is divided into a main light beam and a pair of auxiliary light beams and the main and auxiliary light beams are caused to impinge upon an optical disc-shaped record medium in such a manner that a pair of light beam spots formed on the optical disc-shaped record medium by the auxiliary light beams are positioned in a relation of point symmetry to a light beam spot formed on the optical disc-shaped record medium by the main light beam and two reflected auxiliary light beams emanating from the optical disc-shaped record medium are detected by separate photodetectors, respectively. Detection outputs obtained from the photodetectors by which two reflected auxiliary light beams are detected, respectively, are substantially equal to each other when the main light beam is correctly incident upon a spiral record track on the optical disc-shaped record medium and different from each other when the main light beam is incident upon the optical disc-shaped record medium in an incorrect tracking relation to the spiral record track. Therefore, a tracking error signal is obtained based on a difference between the detection outputs obtained from the photodetectors by which two reflected auxiliary light beams are detected, respectively.
The tracking error signal thus obtained in accordance with the so-called "three-beam system" is formed into a sine-wave signal having a period corresponding to a track pitch defined as a distance between centers of two adjacent turns of the spiral record track on the optical disc-shaped record medium under a condition wherein the light beams incident upon the optical disc-shaped record medium are moved to transverse each turn of the spiral record track. In tracking servocontrol performed with the tracking error signal obtained to have a sinusoidal waveform in accordance with, for example, the so-called "three-beam system", an effective servocontrol range is determined based on a portion of the sinusoidal waveform of the tracking error signal, which varies in substantially linear with a cross point to a DC level between negative and positive peak points of the tracking error signal, and therefore there is a disadvantage that the main light beam incident upon the optical disc-shaped record medium can be maintained in correct tracking relation to the spiral record track only in a relatively narrow effective servocontrol range.